The index finger should be in the middle of the ball. The index finger should be the last finger to roll off the ball on your shot, this is according to Pete Maravich. Maybe that has something to do with it.

I know this is an old thread and the person who posed the initial question is likely long gone, but for anyone else experiencing this problem (sideways rotation), 99 times out of 100 it really is going to be due to the left hand's improper involvement on the shot.

This doesn't mean the left is necessarily participating in the forward push of the ball, but it does likely mean the off-hand is positioned on the ball in a manner that allows the strong arm to set up crookedly, thus creating a sideways counter force against the left hand, which will often result in the aforementioned sideways motion.